October to February is the best time to visit Pune, with November to February the strongest window for general sightseeing.
In these months the weather is dry, clear and comfortable — warm enough during the day for extended sightseeing, cool enough at night to be pleasant. This is the right window for exploring the old city on foot, trekking to the hill forts, and taking day trips into the Ghats without weather constraints.
The monsoon (June to September) is a genuine alternative and, for some visitors, the better choice. The Sahyadri hills are transformed — waterfalls everywhere, the entire range green, and mist over the forts. Lonavala, Khandala, Bhandardara and Tamhini Ghat are at their best. The trade-offs are wet weather, slippery trails and reduced visibility on some days. If landscape and photography matter more to you than convenience, come in the monsoon, ideally in late August or September when the rain is less relentless.
August and September bring Ganesh Chaturthi, which in Pune is a major event. Tilak turned it into a public festival here in the 1890s, and the city's celebrations — particularly around the Dagdusheth temple and the five manache (honoured) Ganpati mandals — are among the most significant in India. Extremely crowded, but a remarkable thing to witness.
March to May is hot but not prohibitive. If you visit, plan outdoor activity for early morning and evening, and consider using Pune as a base for hill stations like Mahabaleshwar and Panchgani, which stay cooler.